1. Field of the Invention
The present invention relates to:
a process and a device for the evaluation of the thermal (or calorific) power of a flow of a combustible gas;
a process and a device for the regulation of the thermal (or calorific) power of a flow of a combustible gas;
a method of characterizing a flowmeter of thermal mass technology.
2. Description of the Related Art
Hot-wire mass flowmeters are well known for industrial or laboratory usage. Their property of offering a wide measurement range makes them particularly attractive: they can be used in measurement ranges varying from 1 to 100. These flowmeters indicate, in a manner known per se, mass or volume flow rates under standard temperature and pressure conditions. This is because their operating principle is based on the measurement of a temperature difference (xcex94xcex8), proportional to the product of the heat capacity (Cp) of the gas, the density (xcfx81) of said gas and the volume flow rate (xcfx86v) of said gas, under standard temperature and pressure conditions:                     Δθ        =                  xe2x80x83                ⁢                  kCp          ⁢                      xe2x80x83                    ⁢                      ρφ            v                                                  =                  xe2x80x83                ⁢                  kCp          ⁢                      xe2x80x83                    ⁢                      φ            m                    ⁢                      xe2x80x83                    ⁢                                    (                                                φ                  m                                :                                  mass                  ⁢                                      xe2x80x83                                    ⁢                  flow                  ⁢                                      xe2x80x83                                    ⁢                  rate                  ⁢                                      xe2x80x83                                    ⁢                  of                  ⁢                                      xe2x80x83                                    ⁢                  said                  ⁢                                      xe2x80x83                                    ⁢                  gas                                            )                        .                              
The use of hot-wire mass flowmeters for measuring gas volume flow rates requires knowledge of, among other things, the density xcfx81 of the gas and the heat capacity Cp of the gas in question. Before it is used, a mass flowmeter must therefore be standardized for the gas whose flow rates will be measured. To measure the flow rate of a second gas, it is necessary to multiply the flow rate indicated by the mass flowmeter by a correction factor F. If the composition of the second gas is close to that of the gas for which the flowmeter has been standardized, the correction factor F may be approximated by xcfx811Cp1 divided by xcfx812Cp2, the index 1 corresponding to the gas for which the flowmeter was standardized and the index 2 corresponding to the second gas for which the flow rate is measured. Thus, in order for a flowmeter to deliver an accurate value of flow rate for a given gas, it is necessary either to standardize it using this gas or to calibrate it by applying a correction factor to the measurement.
In the case of natural gas, this problem is difficult to solve since the chemical composition of the gas varies depending on the location of the point of use and on the source of supply, said source of supply possibly varying over time. The inaccuracies in the volume flow rate measurement may exceed 10% for a flowmeter standardized for pure methane, used with natural gas.
For an industrialist using such a gas as combustible gas, the variations in the chemical composition of said gas are sources of problems since the changes in chemical composition are manifested by changes in calorific power. It is therefore difficult to evaluate and regulate, very accurately, a thermal power (the volume flow rate measured under standard temperature and pressure conditions multiplied by the (net or gross) calorific value) of a flow of gas based on knowing a flow rate given by a mass flowmeter.
The object of the invention is to solve this difficulty by proposing judicious processes and devices making it possible, respectively, to evaluate and regulate the thermal power of a flow of a combustible gas.
The process and devices of the present invention are for the evaluation and the regulation, to a value P0, of the net (or gross) thermal power P of a flow of a combustible gas similar in composition to that of a reference gas, with the flow of gas passing through a flowmeter of thermal mass technology, which is standardized, or standardized and calibrated, for the reference gas. In another embodiment, there is provided a method for calibrating a flowmeter of thermal mass technology.